Optical sensors are sensors of which the sensing principle and optionally the data transfer make use of electromagnetic radiation. Accordingly, optical sensors have a number of advantages over electronic detection systems. Optical sensors are for example more reliable in environments that are difficult to access and/or hazardous to humans, environments such as those found in the oil and gas industry, and are usually not adversely affected by the electromagnetic radiation that is generally produced in for example power cable systems, induction furnaces or equipment for nuclear magnetic resonance measurements, such as MRI or NMR equipment. Other advantages are the easy operation of optical sensors on large distances, their small size, their flexibility and/or the possibility to make a sensor system comprising an array of discrete sensors that all may be read separately from a single optical fibre (a multiplexed sensor system).
Typical sensors that are based on waveguide grating are, e.g., described in detail in U.S. Pat. Nos. 5,380,995, 5,402,231, 5,592,965, 5,841,131, 6,144,026, US 2005/0105841, U.S. Pat. No. 7,038,190, US 2003/156287.
One principle on which such sensors may be based is an axial strain of the waveguide, as a result of an environmental effect that is to be detected, for example by using a coating on the waveguide that deforms under the influence of the environmental effect. An important method via which (a change in) axial strain of the waveguide becomes detectable is to use a grating in the waveguide. When such a grating, guiding a specific spectrum of electromagnetic radiation, stretches or shrinks under the axial strain, the spectral pattern of transmitted radiation and/or the spectral pattern of reflected radiation (i.e. the spectral response) changes. Such changes in the spectral response provide—when measured—quantitative information on the environmental effect. Two examples of a grating are a Fibre Bragg Grating (FBG) and a Long Period Grating (LPG). A multiplexed sensor system can be prepared using a plurality of gratings, in particular FBG's.
For example, US application 2005/0105841 relates to the use of a polyethyleneimine (PEI) monolayer coating on a Long Period Grating waveguide. The coating swells under the uptake of water, which makes a waveguide comprising such coating suitable for measuring relative humidity (RH), based on changes of the refractive index of the coating.
Although it has been widely recognized that optical sensors have a number of advantages over electronic measuring systems, the full potential of optical sensors has not yet been realized. In particular, there is a need for improved sensors for use under extreme conditions, for example under high pressure and/or high temperature. Examples of extreme conditions are conditions that may exist in underground oil or gas reservoirs, or in the equipment that is used to produce oil or gas from these reservoirs. It would in particular be desired to provide an improved sensor for the detection of compounds such as methane, carbon dioxide, hydrogen sulfide or water, e.g. under the conditions as mentioned above. A compound that is to be detected may hereinafter be referred to as ‘analyte’.
WO 03/056313 describes an optical sensor that can operate in offshore environments. The sensor relies on the principle that hydrogen can diffuse into an optical fiber, which results in transmission loss though the fibre at specific wavelengths. The quantity of such a loss is a measure for the amount of hydrogen present in the environment. A limitation of this sensor is that the loss of transmission is generally permanent, because the in-diffused hydrogen causes irreversible changes in the fiber.